TRUE VOLCANOES. 353 



"the mere wreck 51 of one great crater is left "). The beds of 

 lava are not limited and consequently cannot be traced as true 

 streams of small breadth. Tristan da Cuiiha (kt. 37 3' S., 

 long. 1126'W.) discovered as early as 1506 by the Portuguese -, 

 a small circular island of six miles in diameter, in the 

 centre of which a conical mountain is situated, described by 

 Captain Denham as about 8300 feet in height and composed 

 of volcanic rock (Dr. Petermann's Geogr. Mittheil. 1855, 

 No. iii, s. 84). To the south-east, but' in 53 S. latitude, 

 lies the equally volcanic Thompson's Island and between 

 the two in the same direction, Gough Island, &lso called 

 Diego Alvarez. Deception Island, a slender, narrowly 

 opened ring (S. lat. 62 55'), and Bridgeman's Island belong- 

 ing to the South Shetlands group ; both volcanic, with layers 

 of ice, pumice-stone, black ashes and obsidian ; perpetual 

 eruption of hot vapours (Kendal, Journal of the Geographical 

 Society, vol. i, 1831, p. 62). In February, 1842, Deception 

 Island was seen to produce flames simultaneously at 13 

 points in the ring (Dana in United States Explor. Exped. 

 vol. x, p. 548). It is remarkable that, as so many islands in 

 the Atlantic Ocean are volcanic, neither the entire flat islet 

 of Saint Paul 52 (Penedo de S. Pedro), one decree to the north 

 of the equator ; nor the Falklands (with thin quartzose 

 clay-slate), South Georgia or Sandwich land, appear to offer 

 any volcanic rock. On the other hand a region of the 

 Atlantic Ocean, about 20' to the south of the equator, 

 longitude 22 W., is regarded as the seat of a submarine 

 volcano. 53 In this vicinity Krusenstern saw black columns 

 of smoke rise out of the sea (19th May 1806), and in 1836 

 volcanic ashes collected at the same point (south-east from 

 the above mentioned rock of Saint Paul) on two occasions, 



51 Darwin, pp. 84 and 92, with regard to " the great hollow space or 

 valley southward of the central curved ridge, across which the half of 

 the crater must once have extended. It is interesting to trace the steps, 

 by which the structure of a volcanic district becomes obscured and 

 finally obliterated" (See also Scale, Geognosy of the Island of Sain 

 Helena, p. 28). 



52 St. Paul's Rocks. (See Darwin, pp. 3133 and 125). 



53 Daussy on the probable existence of a submarine volcano in the 

 Atlantic, in the Comptes rendus de I' A cad. des Sciences, t. vi, 1858, 

 p. 512 ; Darwin, Volcanic Islands, p. 92 ; Lee, Cruise of the U.S. Brig 

 Dolphin, pp. 255 and 61. 



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